It has been previously proposed in U.S. Pat. No. 4,803,371 to Durland, issued Feb. 7, 1989, to provide an optical scanning method and apparatus for determining the configuration of an elongated object using both non-coherent or "white" light and coherent laser light simultaneously in order to determine both the length of the object and its thickness. In this prior scanner the laser light beams are positioned at different angles with respect to each other and to the optical axis of the photo-detector camera which has the disadvantage that it is difficult to align the laser light beams properly. Also, the laser beam spot on the object changes in position relative to the centerline of the camera field with changes in thickness of the object. These problems are avoided in the present invention by employing laser light beams which are aligned parallel to each other and to the optical axis of the camera which enables them to be more easily aligned and also enables the laser spot position on the object to remain substantially constant in spite of changes in thickness of the object being measured.
It has been found that when using laser light beams which are parallel to the axis of the camera viewing fields, the thickness determinations made from scan data produced by the photo-detector camera for light beams in the inner zone portions of the optical field adjacent the axis of the camera have extremely low accuracy due to the small change in the angle of the reflected laser beams for changes in thickness of the object. However, scan data obtained by the camera for light beams in the outer zone portions of the viewing field more remote from the optical axis produces thickness determinations having a much greater accuracy due to the larger change in angle of the reflected laser beams when striking objects of different thicknesses.
The light scanner apparatus and method of the present invention overcomes the above-discussed problems by employing laser light beams which are positioned parallel to the optical axis of the camera, by interlacing the camera viewing fields to partially overlap the fields of adjacent cameras which are spaced along the entire length of the object and by using only the scan data produced by reflected laser beams in the outer zones of the viewing field remote from the optical axis to measure object thickness changes with greater accuracy. Thus, the scan data corresponding to reflected laser beams in the inner field zones adjacent the optical axis of the camera is only used to measure the thickness of the object when the scan data corresponding to the outer zones is unavailable or defective for a particular camera.
The optical scanning apparatus of the present invention may employ a photoelectric sensor array as the target of the camera in order to convert the light reflected from the surface of the log or other elongated object into electrical scan data signals indicating the thickness of the object. As discussed in the above-referenced Durland patent, the photoelectric sensor may be in the form of a linear diode array including a plurality of PN junction semiconductor diodes which are positioned in a straight line and may be charge coupled together to provide a charge coupled semiconductor device (CCD). This photo-detector sensor measures the thickness of the elongated object by sensing the position of the reflected laser light spot on the linear diode array to indicate such thickness. The position of the light spot on the array corresponds to the distance that the surface of the object is spaced from the camera lens which may be determined mathematically in the manner described in connection with FIG. 4 of the Durland patent. As a result of employing a linear diode array which typically consists of an array of 1024 diodes spaced along a target length of about 0.5" or 13.3 mm so that the size of one individual diode and the change in position of the light spot on such array determine the smallest detectable change in laser dot position on the object and corresponding thickness change of the object. Since the amount of the change in the angle of the reflected laser light beam reflected from the surface of the object to the camera lens due to differences in the thickness of the object determines the change of spot position on the array, a large angle change is extremely important for accurate measurement of the thickness of such object.